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    <h1>
      Two-way Binding Helpers
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    <blockquote>
<p>Note:
<code>LinkedStateMixin</code> is deprecated as of React v15. The recommendation is to explicitly set the value and change handler, instead of using <code>LinkedStateMixin</code>.</p>
</blockquote>

<p><strong>Importing</strong></p>
<div class="highlight"><pre><code class="language-javascript" data-lang="javascript"><span class="kr">import</span> <span class="nx">LinkedStateMixin</span> <span class="nx">from</span> <span class="s1">&#39;react-addons-linked-state-mixin&#39;</span><span class="p">;</span> <span class="c1">// ES6</span>
<span class="kd">var</span> <span class="nx">LinkedStateMixin</span> <span class="o">=</span> <span class="nx">require</span><span class="p">(</span><span class="s1">&#39;react-addons-linked-state-mixin&#39;</span><span class="p">);</span> <span class="c1">// ES5 with npm</span>
</code></pre></div>
<h2>Overview</h2>

<p><code>LinkedStateMixin</code> is an easy way to express two-way binding with React.</p>

<p>In React, data flows one way: from owner to child. This is because data only flows one direction in <a href="https://en.wikipedia.org/wiki/Von_Neumann_architecture">the Von Neumann model of computing</a>. You can think of it as &quot;one-way data binding.&quot;</p>

<p>However, there are lots of applications that require you to read some data and flow it back into your program. For example, when developing forms, you&#39;ll often want to update some React <code>state</code> when you receive user input. Or perhaps you want to perform layout in JavaScript and react to changes in some DOM element size.</p>

<p>In React, you would implement this by listening to a &quot;change&quot; event, read from your data source (usually the DOM) and call <code>setState()</code> on one of your components. &quot;Closing the data flow loop&quot; explicitly leads to more understandable and easier-to-maintain programs. See <a href="/react/docs/forms.html">our forms documentation</a> for more information.</p>

<p>Two-way binding -- implicitly enforcing that some value in the DOM is always consistent with some React <code>state</code> -- is concise and supports a wide variety of applications. We&#39;ve provided <code>LinkedStateMixin</code>: syntactic sugar for setting up the common data flow loop pattern described above, or &quot;linking&quot; some data source to React <code>state</code>.</p>

<blockquote>
<p>Note:</p>

<p><code>LinkedStateMixin</code> is just a thin wrapper and convention around the <code>onChange</code>/<code>setState()</code> pattern. It doesn&#39;t fundamentally change how data flows in your React application.</p>
</blockquote>

<h2>LinkedStateMixin: Before and After</h2>

<p>Here&#39;s a simple form example without using <code>LinkedStateMixin</code>:</p>
<div class="highlight"><pre><code class="language-javascript" data-lang="javascript"><span class="kd">var</span> <span class="nx">createReactClass</span> <span class="o">=</span> <span class="nx">require</span><span class="p">(</span><span class="s1">&#39;create-react-class&#39;</span><span class="p">);</span>

<span class="kd">var</span> <span class="nx">NoLink</span> <span class="o">=</span> <span class="nx">createReactClass</span><span class="p">({</span>
  <span class="nx">getInitialState</span><span class="o">:</span> <span class="kd">function</span><span class="p">()</span> <span class="p">{</span>
    <span class="k">return</span> <span class="p">{</span><span class="nx">message</span><span class="o">:</span> <span class="s1">&#39;Hello!&#39;</span><span class="p">};</span>
  <span class="p">},</span>
  <span class="nx">handleChange</span><span class="o">:</span> <span class="kd">function</span><span class="p">(</span><span class="nx">event</span><span class="p">)</span> <span class="p">{</span>
    <span class="k">this</span><span class="p">.</span><span class="nx">setState</span><span class="p">({</span><span class="nx">message</span><span class="o">:</span> <span class="nx">event</span><span class="p">.</span><span class="nx">target</span><span class="p">.</span><span class="nx">value</span><span class="p">});</span>
  <span class="p">},</span>
  <span class="nx">render</span><span class="o">:</span> <span class="kd">function</span><span class="p">()</span> <span class="p">{</span>
    <span class="kd">var</span> <span class="nx">message</span> <span class="o">=</span> <span class="k">this</span><span class="p">.</span><span class="nx">state</span><span class="p">.</span><span class="nx">message</span><span class="p">;</span>
    <span class="k">return</span> <span class="o">&lt;</span><span class="nx">input</span> <span class="nx">type</span><span class="o">=</span><span class="s2">&quot;text&quot;</span> <span class="nx">value</span><span class="o">=</span><span class="p">{</span><span class="nx">message</span><span class="p">}</span> <span class="nx">onChange</span><span class="o">=</span><span class="p">{</span><span class="k">this</span><span class="p">.</span><span class="nx">handleChange</span><span class="p">}</span> <span class="o">/&gt;</span><span class="p">;</span>
  <span class="p">}</span>
<span class="p">});</span>
</code></pre></div>
<p>This works really well and it&#39;s very clear how data is flowing, however, with a lot of form fields it could get a bit verbose. Let&#39;s use <code>LinkedStateMixin</code> to save us some typing:</p>
<div class="highlight"><pre><code class="language-javascript" data-lang="javascript"><span class="kd">var</span> <span class="nx">createReactClass</span> <span class="o">=</span> <span class="nx">require</span><span class="p">(</span><span class="s1">&#39;create-react-class&#39;</span><span class="p">);</span>

<span class="kd">var</span> <span class="nx">WithLink</span> <span class="o">=</span> <span class="nx">createReactClass</span><span class="p">({</span>
<span class="hll">  <span class="nx">mixins</span><span class="o">:</span> <span class="p">[</span><span class="nx">LinkedStateMixin</span><span class="p">],</span>
</span>  <span class="nx">getInitialState</span><span class="o">:</span> <span class="kd">function</span><span class="p">()</span> <span class="p">{</span>
    <span class="k">return</span> <span class="p">{</span><span class="nx">message</span><span class="o">:</span> <span class="s1">&#39;Hello!&#39;</span><span class="p">};</span>
  <span class="p">},</span>
  <span class="nx">render</span><span class="o">:</span> <span class="kd">function</span><span class="p">()</span> <span class="p">{</span>
<span class="hll">    <span class="k">return</span> <span class="o">&lt;</span><span class="nx">input</span> <span class="nx">type</span><span class="o">=</span><span class="s2">&quot;text&quot;</span> <span class="nx">valueLink</span><span class="o">=</span><span class="p">{</span><span class="k">this</span><span class="p">.</span><span class="nx">linkState</span><span class="p">(</span><span class="s1">&#39;message&#39;</span><span class="p">)}</span> <span class="o">/&gt;</span><span class="p">;</span>
</span>  <span class="p">}</span>
<span class="p">});</span>
</code></pre></div>
<p><code>LinkedStateMixin</code> adds a method to your React component called <code>linkState()</code>. <code>linkState()</code> returns a <code>valueLink</code> object which contains the current value of the React state and a callback to change it.</p>

<p><code>valueLink</code> objects can be passed up and down the tree as props, so it&#39;s easy (and explicit) to set up two-way binding between a component deep in the hierarchy and state that lives higher in the hierarchy.</p>

<p>Note that checkboxes have a special behavior regarding their <code>value</code> attribute, which is the value that will be sent on form submit if the checkbox is checked (defaults to <code>on</code>). The <code>value</code> attribute is not updated when the checkbox is checked or unchecked. For checkboxes, you should use <code>checkedLink</code> instead of <code>valueLink</code>:
<code>
&lt;input type=&quot;checkbox&quot; checkedLink={this.linkState(&#39;booleanValue&#39;)} /&gt;
</code></p>

<h2>Under the Hood</h2>

<p>There are two sides to <code>LinkedStateMixin</code>: the place where you create the <code>valueLink</code> instance and the place where you use it. To prove how simple <code>LinkedStateMixin</code> is, let&#39;s rewrite each side separately to be more explicit.</p>

<h3>valueLink Without LinkedStateMixin</h3>
<div class="highlight"><pre><code class="language-javascript" data-lang="javascript"><span class="kd">var</span> <span class="nx">createReactClass</span> <span class="o">=</span> <span class="nx">require</span><span class="p">(</span><span class="s1">&#39;create-react-class&#39;</span><span class="p">);</span>

<span class="kd">var</span> <span class="nx">WithoutMixin</span> <span class="o">=</span> <span class="nx">createReactClass</span><span class="p">({</span>
  <span class="nx">getInitialState</span><span class="o">:</span> <span class="kd">function</span><span class="p">()</span> <span class="p">{</span>
    <span class="k">return</span> <span class="p">{</span><span class="nx">message</span><span class="o">:</span> <span class="s1">&#39;Hello!&#39;</span><span class="p">};</span>
  <span class="p">},</span>
<span class="hll">  <span class="nx">handleChange</span><span class="o">:</span> <span class="kd">function</span><span class="p">(</span><span class="nx">newValue</span><span class="p">)</span> <span class="p">{</span>
</span><span class="hll">    <span class="k">this</span><span class="p">.</span><span class="nx">setState</span><span class="p">({</span><span class="nx">message</span><span class="o">:</span> <span class="nx">newValue</span><span class="p">});</span>
</span><span class="hll">  <span class="p">},</span>
</span>  <span class="nx">render</span><span class="o">:</span> <span class="kd">function</span><span class="p">()</span> <span class="p">{</span>
<span class="hll">    <span class="kd">var</span> <span class="nx">valueLink</span> <span class="o">=</span> <span class="p">{</span>
</span><span class="hll">      <span class="nx">value</span><span class="o">:</span> <span class="k">this</span><span class="p">.</span><span class="nx">state</span><span class="p">.</span><span class="nx">message</span><span class="p">,</span>
</span><span class="hll">      <span class="nx">requestChange</span><span class="o">:</span> <span class="k">this</span><span class="p">.</span><span class="nx">handleChange</span>
</span><span class="hll">    <span class="p">};</span>
</span>    <span class="k">return</span> <span class="o">&lt;</span><span class="nx">input</span> <span class="nx">type</span><span class="o">=</span><span class="s2">&quot;text&quot;</span> <span class="nx">valueLink</span><span class="o">=</span><span class="p">{</span><span class="nx">valueLink</span><span class="p">}</span> <span class="o">/&gt;</span><span class="p">;</span>
  <span class="p">}</span>
<span class="p">});</span>
</code></pre></div>
<p>As you can see, <code>valueLink</code> objects are very simple objects that just have a <code>value</code> and <code>requestChange</code> prop. And <code>LinkedStateMixin</code> is similarly simple: it just populates those fields with a value from <code>this.state</code> and a callback that calls <code>this.setState()</code>.</p>

<h3>LinkedStateMixin Without valueLink</h3>
<div class="highlight"><pre><code class="language-javascript" data-lang="javascript"><span class="kd">var</span> <span class="nx">LinkedStateMixin</span> <span class="o">=</span> <span class="nx">require</span><span class="p">(</span><span class="s1">&#39;react-addons-linked-state-mixin&#39;</span><span class="p">);</span>
<span class="kd">var</span> <span class="nx">createReactClass</span> <span class="o">=</span> <span class="nx">require</span><span class="p">(</span><span class="s1">&#39;create-react-class&#39;</span><span class="p">);</span>

<span class="kd">var</span> <span class="nx">WithoutLink</span> <span class="o">=</span> <span class="nx">createReactClass</span><span class="p">({</span>
  <span class="nx">mixins</span><span class="o">:</span> <span class="p">[</span><span class="nx">LinkedStateMixin</span><span class="p">],</span>
  <span class="nx">getInitialState</span><span class="o">:</span> <span class="kd">function</span><span class="p">()</span> <span class="p">{</span>
    <span class="k">return</span> <span class="p">{</span><span class="nx">message</span><span class="o">:</span> <span class="s1">&#39;Hello!&#39;</span><span class="p">};</span>
  <span class="p">},</span>
  <span class="nx">render</span><span class="o">:</span> <span class="kd">function</span><span class="p">()</span> <span class="p">{</span>
    <span class="kd">var</span> <span class="nx">valueLink</span> <span class="o">=</span> <span class="k">this</span><span class="p">.</span><span class="nx">linkState</span><span class="p">(</span><span class="s1">&#39;message&#39;</span><span class="p">);</span>
    <span class="kd">var</span> <span class="nx">handleChange</span> <span class="o">=</span> <span class="kd">function</span><span class="p">(</span><span class="nx">e</span><span class="p">)</span> <span class="p">{</span>
      <span class="nx">valueLink</span><span class="p">.</span><span class="nx">requestChange</span><span class="p">(</span><span class="nx">e</span><span class="p">.</span><span class="nx">target</span><span class="p">.</span><span class="nx">value</span><span class="p">);</span>
    <span class="p">};</span>
    <span class="k">return</span> <span class="o">&lt;</span><span class="nx">input</span> <span class="nx">type</span><span class="o">=</span><span class="s2">&quot;text&quot;</span> <span class="nx">value</span><span class="o">=</span><span class="p">{</span><span class="nx">valueLink</span><span class="p">.</span><span class="nx">value</span><span class="p">}</span> <span class="nx">onChange</span><span class="o">=</span><span class="p">{</span><span class="nx">handleChange</span><span class="p">}</span> <span class="o">/&gt;</span><span class="p">;</span>
  <span class="p">}</span>
<span class="p">});</span>
</code></pre></div>
<p>The <code>valueLink</code> prop is also quite simple. It simply handles the <code>onChange</code> event and calls <code>this.props.valueLink.requestChange()</code> and also uses <code>this.props.valueLink.value</code> instead of <code>this.props.value</code>. That&#39;s it!</p>


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